Electric heater terminal



Dec. 13, 1955 NELSON ET AL 2,727,121

ELECTRIC HEATER TERMINAL Filed Sept. 24, 1954 lnveMors:

Joseph F. Nelson Roberf W. Sawyer by. Wand:

Their AHorney United States Patent G ELECTRIC nna'raa TERMINAL Joseph F.Nelson, Pittsfield, and Robert W. Sawyer, Lee, Mass, assignors toGeneral Electric (Jempany, a can poration of New York ApplicationSeptember 24, 1954, Serial No. ddthwti 4 Claims. (Cl. 201-67) Thisinvention relates to terminal structures and has particular applicationto terminal structures for electrical resistance heaters of the encasedor sheathed type wherein a resistance heating element is embedded in acompacted mass of electrically insulating, heat conducting materialwhich is, in turn, enclosed by an outer metallic sheath.

It is well known that, in heaters of the above type, it is extremelydesirable from the standpoint of obtaining an adequate operating life,that the resistance heating element and its associated heat conductingand electrically insulating material be hermetically sealed within theheater from the deleterious effects of moisture and various harmfulatmospheres and gases in which the heater may be required to operate. Itwill be seen that, with the well known sheathed heater constructionwherein an outer sheath of non-porous material is employed, the

problem of hermetically sealing the heater is concerned principally withproviding a terminal structure in which a hermetic seal is eifectedaround the corresponding terminal opening of the heater.

it is accordingly one object of this invention to provide an improvedterminal structure for heaters of the encased or sheathed type whichterminal structure is of a rugged and inexpensive construction, andwhich provides a hermetic seal around the corresponding terminal openingof the heater.

Briefly stated, this invention, in accordance with one aspect thereof,employs a sealing member of non-porous ceramic material and provides aconstruction utilizing butt joints between the ceramic sealing memberand the adjacent metallic members to which it is attached in sealingrelationship in any suitable manner such as by means of a titaniumhydride braze. The requirement for holding close tolerances in themanufacture of the ceramic sealing member is thus eliminated while, atthe same time, a mechanically rugged and hermetically sealedconstruction is provided. In addition, this invention provides, inaccordance with one embodiment thereof, a terminal structure whichprovides a hermetic seal around the terminal opening without exceedingthe diameter of the heater sheath. Further, there is provided in oneembodiment an arrangement for minimizing mechanical stresses which mightotherwise be induced by differences in the temperature coefficients ofexpansion of the adjoining metal and ceramic parts.

Other objects and advantages of this invention will be apparent from thefollowing description taken in connection with the accompanyingdrawings, and its scope will be pointed out in the appended claims.

Referring to the drawing, Fig. 1 is a perspective view, shown partly incross section, of a sheathed heater embodying this invention with one ofthe terminals being shown in an exploded view to more clearly illustratethe elements of the terminal structure; while Fig. 2 is a perspectiveview, also shown partly in cross section with an exploded view of one ofthe terminals, of a sheathed heater embodying this invention andillustrating an arrangement alternative to that shown in Fig. 1.

2,727,121 Patented Dec. 13, 1955 As illustrated in Fig. 1, thisinvention is particularly applicable to electric resistance heaters ofthe encased or sheathed type wherein a resistance heating element 1 isembedded in a compacted mass of electrically insulating and heatconducting material 2 which is, in turn enclosed by an outer metallicsheath 3. The heat conducting, electrically insulating material 2 may bea material such as magnesium oxide, which can be loaded into the sheath3 in powdered form around the heating element 1 and then compacted intoa dense mass by means of elongating and reducing the diameter of thesheath 3, usually by swaging or rolling.

in order to ensure that the protective layer of heat conducting andelectrically insulating material 2 remains in good condition during thelife of the heater, it is very desirable that this material behermetically sealed within the heater so that it does not become exposedto the deleterious action of moisture and certain harmful atmospheres inwhich the heater may be required to operate. Thus, with both theresistance heating element and the surrounding heat conducting,electrically insulating material sealed within the heater enclosure, theoperating life of the heater can be considerably extended.

As has been pointed out, one of the most difficult areas with respect toproviding a hermetically sealed heater of the aforementioned type, liesin the problem of sealing the terminal openings of the heater while atthe same time providing an inexpensive and mechanically rugged terminalstructure which is physically compact in size.

Referring now to the terminal structure illustrated in Fig. l andembodying this invention, it will be seen that the heater shown isprovided with a pair of terminal membets 4 which extend into and makeelectrical contact with the opposite ends of the heating element 1, andwhich extend out from the ends of the tubular sheath 3. For purposes ofdescription, the elements illustrated in the exploded view at the righthand portion of Fig. 1 are represented by the same reference numbers aslike elements shown in the assembled terminal structure at the left handend of the heater of Fig. 1.

Referring now to the assembled terminal structure at the left hand endof the heater of Fig. 1, the open end of the sheath 3 is sealed by meansof an assembly comprising a non-porous, electrically insulating disk 5of ceramic or other heat refractory material and a metallic cover disk6. The ceramic disk 5 is attached in hermetically sealing relationshipto the edge portion of the sheath 3 by means of any suitable method forattaching ceramic to metal, such as by means of a titanium hydride brazeor by the titanium shim method, which is the method illustrated in theexploded view at the right hand portion of the heater of Fig. 1.

The sealing disk 5 may be formed of any suitable nonporous, electricallyinsulating and heat refractory material such as a ceramic material. Byway of example, it has been found that the disk 5 will performsatisfactorily it formed of a ceramic material commonly known as aluminaceramic, which is composed predominately of aluminum oxide with smallpercentages of clay and talc acting as binder and fiux respectively.

The disk 5 is provided with a central aperture 7 through which theterminal member 4 extends. The aperture 7 may be considerably largerthan the diameter of the terminal member 4 since, as will be seen fromthe following description, it is not necessary that a seal be effectedbetween the disk 5 and the terminal member 4 along the diameter of theterminal member 4.

The seal between the terminal member 4 and the ceramic disk 5 iseffected by means of the cover disk 6 which is brazed or otherwisesuitably attached around an aperture 6 therein to the terminal member 4,and which is also attached in sealing relationship to the abutting faceof the ceramic disk 5 by means of any suitable method of attaching metalto ceramic such as discussed above.

It will be seen that with the relationship of parts just describedallsealed joints or connections between the ceramic disk 5 and theadjoining metal parts are made by means of butt joints and that,consequently, any requirement for holding close tolerances on theceramic disk 5 is eliminated. It is therefore possible to mold or castthe ceramic disk 5 directly into its final shape so that it can beutilized in the assembly of the terminal structure without any furthermachine or grinding operations.

In addition, it will be observed that by attaching the cover plate 6 tothe terminal member 4 by means of brazing or by some other processinvolving heating of the terminal structure to a fairly hightemperature, the ceramic disk is then put under axial compression uponcooling of the terminal assembly by reason of the correspondingcontraction of the terminal member 4 along the axial directioh. Theaforementioned axial compressive force results from the fact that thecoefiicient of thermal expansion of the material of the terminal member4 is greater than the thermal coeflicie'nt of expansion of the ceramicmember 5. Therefore, when the assembly is heated in the brazingoperation, the terminal member 4 expands outwardly in the axialdirection by a greater amount than does the ceramic disk 5. Then, uponcooling of the assembly, the brazed joint between the cover disk 6 andthe terminal member 4 solidifies to form a solid connection and furthercooling results in compression of the ceramic disk 5.

The ceramic disk 5 remains under a compressive force even when theterminal reaches normal operating temperature since, for mostapplications, the operating temperature of the terminal structure wouldnot exceed 400 F. or so whereas the brazing operation is normallycarried out at temperatures in the vicinity of 1100 F. to l500 F.Consequently, even at normal operating temperature, the terminalassembly is still considerably cooler than the brazing temperature sothat the ceramic disk 5 remains in compression.

The fact that the ceramic disk 5 is continuously under a compressiveforce results in a more rugged terminal structure since, as is wellknown, ceramic materials are relatively strong in compression but quiteweak in tension so that it is desirable for that reason to place theceramic material under compression and thereby provide a prestressedcondition. The net result is that the terminal structure just describedis less subject to mechanical failure since, before any tensile stressescan be induced, the initial compressive stresses must first be overcome.

The exploded View at the right hand of Fig. l is inchided to show onemethod of manufacturing the heater terminal structure illustrated at theleft hand end of the heater of Fig. l. The method illustrated includesthe utilization of the titanium shim method for attaching metal toceramic, which method comprises clamping the metal and ceramic partstogether with a thin titanium shim therebetween and then heating theclamped assembly to a high temperature either in a vacuum or controlledatmosphere to effect a bond between the parts.

In manufacturing the terminal assembly of the heater shown in Fig. l inaccordance with the titanium shim method just discussed, one procedurewhich has been found to be satisfactory comprises clamping the elements5 and 6 over the terminal 4 to the terminal end of the sheath 3 with atitanium shim 8 between the elements 5 and 6 and a second titanium shim9 between the ceramic member 5 and the sheath 3. The clamped assembly isthen heated in a vacuum or controlled atmosphere to effect a bondbetween the ceramic and metal parts as described above. In the sameoperation, the cover disk 6 is brazed or otherwise suitably attached tothe terminal member 4, thereby completing the terminal structure andeffecting the seal in a single heating operation.

In a second embodiment of this invention, which is illustrated in Fig.2, provision is made for compensating for possible differences inthermal coefiicients of expansion between the metal of the sheath andthe ceramic disk. The advantages inherent in the construction of Fig. lare, of course, present in the construction illustrated in Fig. 2 inthat there is no requirement for close tolerances in the manufacture ofthe ceramic sealing disk and further in that a rugged, inexpensive andhermetically sealed terminal structure is provided. It will be notedalso that the ceramic sealing member 5 of the construction of Fig. 2 isplaced under a pre-stressed compressive force as described in connectionwith the structure of Fig. 1 thereby yielding the advantages discussedabove. The construction of Fig. 2 therefore is illustrated to show analternative arrangement which may be advantageously employed in caseswhere, by reason of certain limitations on selection of materials,problems of differences in thermal coefficients of expansion of theceramic sealing disk and the metallic sheath exist. 7

Referring now to the assembled terminal structure at the left hand endof the heater of Fig. 2, in which the elements thereof are identified bythe same reference numerals as like elements in the construction of Fig.l, the construction illustrated is similar to that of Fig. 1 except thata cylindrical ring 10 of metallic material is provided interconnectingthe ceramic sealing disk 5 and the sheath 3. The cylindrical ring 10 isof tubular form and may have a cross sectional shape substantially thesame as that of the sheath 3 so that when the ring 10 is attached to thesheath 3 it forms in shape an extension thereof. The cylinder 10 is of ametallic material which has a thermal coefiicient of expansion closelycorresponding to the coefiicient of expansion of the ceramic disk 5 or,in the alternative, it may be of a pure ductile metal, such as purecopper, iron or nickel, which, although not having a coeflicient ofexpansion closely corresponding to that of the ceramic disk 5, providesa yielding connection between the disk 5 and the sheath 3, therebyavoiding the occurrence of any high mechanical stresses by reason ofdifferences in thermal expansion characteristics.

With this construction it will be seen that if the metal which forms thecylindrical ring 10 is selected so as to have a thermal coefficient ofexpansion closely corresponding to that of the ceramic disk 5, thedifference in physical expansion between the disk 5 and the sheath 3will appear at the joint between the metallic ring 10 and the sheath 3,which joint can be much stronger than the connection between themetallic ring 10 and the disk 5 so that the mechanical stresses inducedby differences in thermal coeflicients of expansion do not reach anobjectionable level.

In the event that the disk 5 is composed of alumina ceramic, which, asstated above, is composed predominately of aluminum oxide, the metallicring 10 may be manufactured of a metallic material identified as a 14%nickel-iron alloy which has expansion characteristics similar to thoseof alumina ceramic up to about 800 F. Good results may also be obtainedif, as mentioned above, the ring 10 is manufactured of a pure ductilemetal such as copper, iron, or nickel in a pure or substantially purestate.

It may be found convenient to manufacture the thermal assemblyillustrated in Fig. 2 in a similar manner to that described inconnection with the structure of Fig. l, or it may be found moredesirable to employ a subassembly method. In such a method, thesubassembly indicated by the bracket a, which comprises the metallicring 10, the ceramic sealing member 5, and the cover plate 6, is firstformed by utilizing the titanium hydride process, the titanium shimmethod many other suitable method or process for attaching metal andceramic materials. In the event that it is desired to employ thetitanium shim method, the elements 10, 5 and 6 are first clampedtogether with the intermediate titanium shims 8 and 9 as illustrated.This subassembly is then heated to a high temperature in a vacuum orprotective atmosphere in order to eifect a hermetically sealed bondbetween the abutting ceramic and metal surfaces.

When the subassembly a is formed, it is then inserted over the terminalmember 4 with the edge of the metallic member 19 being brought intoabutting relationship with the edge of the sheath 3 and the terminalmember 4 extending through the apertures 7 and 6 in the members 5 and 6respectively. The member 10 is then brazed or otherwise attached to thesheath 3 to form a hermetically sealed connection therewith and, in thesame operation, the cover disk 6 is then affixed to the terminal member4 by means of brazing or some other suitable process.

It will be realized of course that in the construction of Fig. 1 thesheath 3 may be formed entirely out of a material having thermalexpansion characteristics similar to those of the ceramic disk 5 or, onthe other hand, the sheath 3 may be formed of a ductile material inorder to prevent the occurrence of high mechanical stresses which mightotherwise be induced by differences in thermal expansion characteristicsbetween the abutting metal and ceramic materials. it will also berealized that in certain applications, such as in those where theoperating temperature of the heater is relatively low, the problem ofdiiferences in thermal expansion between the metal and ceramic materialsmay not arise, in which case the construction of Fig. 1 may beadvantageously employed without any special restrictions on theselection of materials.

It will be understood that the terminal structure of the presentinvention may be manufactured in other ways than in the particularmanner illustrated and described herein and that the present method ofmanufacture is set forth as representative of one method which has beenfound to be satisfactory. In addition, the ceramic and metal parts maybe attached together by means other than the titanium shim method whichis set forth herein for purposes of illustration only.

It will be apparent from the foregoing that various modifications,changes, substitutions and combinations may be employed in accordancewith the teachings set forth herein without departing from the scope ofthis invention in its broader aspects as defined in the appended claims.

What We claim as new and desire to secure by Letters Patent of theUnited States is:

1. A terminal structure for a tubular sheathed heater comprising ametallic terminal member extending from one end of said sheath, atubular member formed of a metallic material and having a crosssectional shape sulvstantially the same as that of said sheath, saidtubular metallic member being attached in sealing relationship to theedge of said sheath so as to form in shape an extension thereof, anon-porous ceramic sealing member shaped so as to abut against the edgeof said tubular member opposite the edge affixed to said sheath, saidceramic sealing member being attached in abutting relationship to thesaid opposite edge of said tubular member and having an opening thereinthrough which said terminal member extends, and a metallic cover membertached in abutting relationship to said ceramic member on the sidethereof opposite from the side attached to said tubular member, saidcover member extending around said terminal member and being attached insealing relationship thereto.

2. A terminal structure for a sheathed heater comprising a terminalmember for said heater extending from one end of said sheath, a tubularmember formed of a substantially pure ductile metal and having a crosssectional shape substantially the same as that of said sheath, saidtubular member being attached in sealing relationship to the edge ofsaid sheath so as to form in shape an extension thereof, a non-porousceramic sealing member shaped so as to abut against the edge of saidtubular member opposite the edge afiixed to said sheath, said ceramicsealing member being attached in abutting relationship to the saidopposite edge of said tubular member and having an opening thereinthrough which said terminal member extends, said tubular member therebyproviding a yieldable sealed connection between said ceramic member andsaid sheath and acting to reduce mechanical stresses occurring by reasonof differences in thermal coefiicients of expansion between said sheathand said ceramic member, and a metallic cover member attached inabutting relationship to said ceramic member on the side thereofopposite from the side attached to said tubular member, said covermember extending around said terminal member and being attached insealing relationship thereto.

3. A terminal structure for a tubular sheathed heater comprising ametallic terminal member for said heater extending from one end of saidsheath, a tubular member formed of a metallic material and having across sectional shape substantially the same as that of said sheath,said tubular member being attached to and extending from the terminaledge of said sheath, a non-porous ceramic sealing member shaped so as toabut against the edge of said tubular member opposite the edge aflixedto said sheath, said ceramic sealing member being attached in abuttingrelationship to the said opposite edge of said tubular member and havingan opening therein through which said terminal member extends, saidtubular member and said ceramic sealing member having closelycorresponding thermal expansion characteristics over a substantialportion of the range of operating temperatures for minimizing mechanicalstresses induced by reason of differences in said thermal expansioncharacteristics, and a metallic cover member attached in abuttingrelationship to said ceramic member on the side thereof opposite fromthe side attached to said tubular member, said cover member extendingaround said terminal member and being attached in sealing relationshipthereto.

4. A terminal structure for a tubular sheathed heater comprising ametallic terminal member extending from one end of said sheath, atubular member formed of a metallic material attached in sealingrelationship to the edge of said sheath so as to form in shape anextension thereof, a non-porous ceramic sealing member shaped so as toabut against the edge of said tubular member opposite the edge afiixedto said sheath, said ceramic sealing member being attached in abuttingrelationship to said opposite edge of said tubular member and having anopening therein through which said terminal member extends, and ametallic cover member attached in abutting relationship to said ceramicmember on the side thereof opposite from the side attached to saidtubular member, said cover member extending around said terminal memberand being attached in sealing relationship thereto.

References Cited in the file of this patent UNITED STATES PATENTS2,003,175 Daly May 28, 1935

